1. Field of the Invention
This invention relates to a molding apparatus for molding articles by introducing molten resin into its mold cavity.
2. Description of Related Art
In a typical process of injection molding, molten resin introduced through a gate of a mold flows along an inner face of a mold cavity. When the mold is formed with a single gate, the molten resin flows from the gate, as a center, radially. Depending on the mold structure, the stress of a molded article is localized near the gate, and the material density of the molded article decreases as its position approaches to the periphery of the molded article. If the mold has a plate-shaped cavity, such as for a gear, and also has a center hole, the gate of the mold can not be positioned at the center of the cavity. In FIG. 1, a mold for producing a gear is shown. Since the gear has a center hole 61, three gates 62a, 62b, and 62c, which are equally spaced with each other, introduce molten resin into the cavity.
The localized stress in the molded articles affects the quality of the molded article. When the plate-shaped mold cavity is used for optical disc recording medium and the like, stress in the molded articles causes disunity of birefringent ratio, thereby causing failure of the molded articles.
In FIG. 1, since molten resin is filled from each of gates 62a, 62b, and 62c, each of areas 63a, 63b, and 63c extending radially from the corresponding gates has high material density. In areas 64d, 64e, and 64f outside of the areas 63a, 63b, and 63c, however, molten resin is filled so that material density of those areas is inevitably low. In order to reduce the low density areas 64d, 64e, and 64f, a multiple gate may be effective, but the multiple gate puts the design of the mold under constraints.
When molten resin is introduced into a plate-shaped cavity, such as for an optical lens, through a single gate located at the outer periphery of the cavity, weld lines tend to occur at the area at which edges of molten material flows which are split once are joined again.
By analyses of filling of molten resin into the mold cavity, when molten resin flows from the gate into the cavity, the molten resin starts to flow from a mold face to which the molten resin firstly contacts. The molten resin spreads from the mold face to which the molten resin firstly contacts, as a center, to other section of the mold face. In other words, since the gate is fixed, the molten resin of each injection dose not always flow on the same mold face. Therefore, disunity of the molded articles occurs.